Otter Adaptations:
How Do Otters Remain Sleek and WarmOr, What One Cell Biologist Does Away From the Bench

(Philadelphia, PA) – Otters cavorting in the
water is a scene with which we’re all familiar.
Yet, unlike many other mammals that spend a considerable
amount of time in the water–polar bears, seals,
dolphins, and whales–river otters do not have
a thick layer of body fat to keep warm. They rely, instead,
on a few unique adaptations; namely, their fur and the
densely packed layer of specially adapted underhairs.

Using
scanning electron microscopy and polarizing light microsopy,
John W. Weisel, PhD, Professor of Cell
and Developmental Biology at the University
of Pennsylvania School of Medicine, and colleagues,
examined the structure of these hairs for clues to their
exceptional insulation abilities. (Click on the thumbnail
to view full-size images). They found that the cuticle
surface structure of the underhairs and base of the
less-abundant guard hairs are distinctively shaped to
interlock, with wedge-shaped fins or petals fitting
into wedge-shaped grooves between fins of adjacent hairs.
Weisel and colleagues report their findings in the Canadian
Journal of Zoology.

Weisel and Research Specialist Chandrasekaran
Nagaswami, MD, also in Penn’s Department
of Cell and Developmental Biology, usually work on defining
the physical properties of blood clots and applying
this knowledge to find better treatments for heart disease.
Two years ago when Weisel, an avid hiker, climber, and
white-water kayaker, took a month of his sabbatical
year to study wolves–a life-long interest–on
Isle Royale National Park in Lake Superior, Michigan,
he also collected hair samples from the island’s
mammals—including wolves, moose, and otters. (The
ecological studies of wolves and moose on Isle Royale,
which started in 1959, are part of the longest-running
animal ecology study in the world. Isle Royale has been
a training ground for many ecologists, and lessons learned
here have been applied to the re-introduction of wolves
to Yellowstone National Park.)

Weisel examined wolf prey hair with light and electron
microscopy with the idea of accurately identifying wolf
diet from wolf scat. “While we have engaged molecular
biologists in studies of animal genetics and isotope
dynamics, John is the first structural molecular biologist
that we have worked with,” says wildlife biologist
Rolf Peterson from Michigan Technological University
(Houghton, Mich.), who has spent the last three decades
doing field research on Isle Royale. “It was a
delight to learn about important basic features of animal
hair that facilitate their unique lifestyles.”

“Most hair from animals has a distinctive pattern,
which is how we can distinguish one species from another,”
says Weisel. “But otter hair is so different that
it caught my attention.” The fins of one hair
loosely insert into the grooves between fins of an adjacent
hair, thus permitting the hairs to form a web-like pattern
that keeps water from the otter’s skin and decreases
heat loss. Also, the grooves between fins trap air bubbles,
which help increase the thermal insulation of the otter’s
coat. Indeed, biologists have observed otters actively
blowing air bubbles into their fur while grooming, and
their energetic rolling catches air in their fur. “The
air insulates like a down jacket,” explains Weisel.

A common otter behavior, next to their playfulness,
is their constant grooming. This behavior is another
important aspect of an otter’s heat-sparing abilities.
In addition to the interlocking structure of the underhairs,
these hairs are coated with a thin layer of body oil
from the otter’s sebaceous glands, thus providing
another barrier to water. The fins of the underhairs
are also aligned away from the body, which is consistent
with the direction in which otters run their paws through
their hair during this self-grooming, thereby ensuring
that their claws do not get caught on the fin-like projections.

Weisel is continuing these studies of mammal hair in
his spare time and has returned to Isle Royale once
since his sabbatical, doing radio telemetry of radio-collared
wolves and collecting samples of their scat for DNA
analysis.

“I discovered that it can be very enjoyable and
stimulating to expand your scientific horizons beyond
the familiar, and even get to take a ‘busman’s
holiday’ in a beautiful place with wonderful people,
enriching your scientific and personal life,”
says Weisel of his experiences away from the bench.
“There are still a great many new things to learn,
but some approaches and ideas from one field can be
useful in another.”

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